The present invention relates to electrical circuit switches. More particularly, the present invention relates to bipolar junction transistor switches.
Bipolar Junction Transistors (BJT) are transistors commonly utilized as amplifiers, switches, and oscillators. The operation of BJTs relies on charge carrier movement from an emitter region to a collector region via a base region. The emitter regions are commonly doped with a first conductivity type, and the base region doped with a second conductivity type. For a BJT where the first conductivity type is n-type and the second conductivity type is p-type, during operation, the emitter region is forward biased to drive electron carriers from the n-type emitter through the p-type base region to the collector region, while the collector region is reverse biased to prevent electron carriers from travelling from the collector region to the base region.
In a semiconductor, electron carriers (negative charge carriers) move about in the conduction band and hole carriers (positive charge carriers) move about in the valence band. In the space-charge region of a p-n junction, for example the space-charge region of base-collector junction of a BJT, there is an electric field due to the voltage differential between the p-type region and the n-type region of the p-n junction. The larger the voltage differential, the larger the electric field. When electron and hole carriers move about in a space-charge region with electric field, they can gain enough energy from the electric field to dislodge fixed electrons (non-carrier electrons), lifting them from the valence band to the conduction band. Once lifted from the valence band to the conduction band, the dislodged electrons are electron carriers and are free to move about in the conduction band. For each electron thus lifted from the valence band to the conduction band, a hole carrier is created in the valence band. The hole carriers thus created are free to move about in the valence band. The process in which a high-energy electron or hole carrier dislodges a fixed electron to create an electron carrier in the conduction band and a hole carrier in the valence band is known as impact ionization. At high enough voltage between the p-type region and the n-type region, the electron and hole carriers created in the impact ionization process can themselves undergo impact ionization processes and create more electron and hole carriers. At still higher voltage between the p-type region and the n-type region, the cascading impact ionization processes multiply into what is commonly known as avalanche breakdown.
For certain applications, such as power and motor switches, BJTs often require relatively high voltages across the emitter and collector regions. The voltage across the emitter and collector regions at which avalanche breakdown occurs in the base-collector junction is the collector to emitter breakdown voltage (BVCEO). For reliable operation, the voltage across the emitter and collector regions must be less than the breakdown voltage BVCEO of the BJT.
In order to address avalanche breakdown in such bipolar junction transistor applications, a common approach is to lightly dope the collector region. For a given voltage across a p-n junction, the electric field is smaller when the dopant concentration of the p-type region, the n-type region, or both the p-type and n-type regions, is reduced. Thus, a BJT having a lightly doped collector region has a larger collector emitter breakdown voltage than a BJT having a more heavily doped collector region. However, a lightly doped collector region also means low charge carrier density in the collector region, and hence low current carrying capability for the BJT.
In the operation of a BJT, charge carriers are injected from the emitter region into the base region. As these charge carriers traverse the base region and reach the collector region, they become the charge carriers in the collector region. A BJT operated to carry a large current implies a large density of charge carriers injected from the emitter region and reaching the collector region. When the density of charge carriers in the collector, injected from the emitter, is larger than the dopant density of the collector region, there is a corresponding rise in the density of charge carriers of the opposite polarity in the collector in order to maintain local charge neutrality in the collector region. (Charge neutrality means there is no net charge, positive or negative, present in the region.) In other words, when the density of charge carriers in the collector, injected from the emitter, is larger than the dopant density of the collector region, the BJT behaves as if a corresponding amount of charge carriers of the opposite polarity from the base are pushed into, and accumulate in, the collector region. This phenomenon, commonly referred to as the Kirk Effect or base pushout, increases transit times (particularly on-to-off-time). Therefore, in conventional BJT design, there is a design conflict between large current carrying capability which requires high collector dopant density, and large collector-emitter breakdown voltage which requires low collector dopant density. As such, the conventional BJT is simply inferior for high speed, high voltage applications, such as power and motor switches.